Land and ocean temperatures for the period of January to June were the hottest seen since record-keeping began in 1880, according to an analysis released July 15 by the U.S. National Oceanic and Atmospheric Administration (NOAA).

The average temperature for the first half of 2010 was 57.5 degrees Fahrenheit (14.2 degrees Celsius)—about 1.2 degrees Fahrenheit (0.7 degrees Celsius) above the 20th-century average.

Nine countries shattered heat records, including Pakistan, which on May 26 logged a mercury reading of 128.3 degrees Fahrenheit (53.5 degrees Celsius)—the highest ever seen in Asia, according to Jeff Masters, director of meteorology for the Weather Underground website.

While some regions heated up—such as Asia, Peru, and the eastern U.S.—short-term climate impacts meant other areas saw their coolest temperatures yet.

Southern China's Guizhou Province, for instance, experienced its coolest June on record, the report said.

"It's too early to extrapolate and say it's the hottest" year ever recorded—a title currently held by 2005. But 2010 "will almost certainly be at least the third or fourth warmest on record," said Derek Arndt, head of the Climate Monitoring Branch of NOAA's National Climatic Data Center.

El Niño Gives Global Warming a "Nudge"

Part of the reason 2010 likely won't be the hottest year on record is due to the interaction of two powerful climate phenomena, El Niño and La Niña.

El Niño is a warming of tropical waters in the central and eastern Pacific Ocean. During El Niño years, the warmer currents act as a "nudge" that heats the planet on top of the steady warming trend caused by human-induced greenhouse gases, according to Arndt. (Test your global warming knowledge.)

But every three to seven years, El Niño alternates with La Niña, a cooling of the eastern tropical Pacific.

In late May El Niño dissipated, clearing the way for La Niña, which for the rest of 2010 could counteract the warming set in motion by El Niño, Arndt said.

It's likely, however, that El Niño will have lingering warming effects through August, noted Kevin Trenberth, a senior scientist at the Boulder, Colorado-based National Center for Atmospheric Research.

Overall, the long-term warming trend can be compared to riding up an escalator, Arndt added, while natural variables such as El Niño and La Niña are akin to a person jumping up and down during the steady ascent.

Trenberth agreed that El Niño and La Niña represent "wiggles," and "when they're on top of a rising trend, that's when you break records."

"It's not just global warming and not just natural variability—it's a combination of both."

Since 1992 satellites have revealed that sea level has risen about 2.2 inches (5.6 centimeters)—a rate that is equivalent to a foot (0.3 meter) or more of sea-level increase a century, Trenberth said.

The new NOAA report also found that, in June 2010, Arctic sea ice covered about 4.2 million square miles (10.9 million square kilometers), which is 10.6 percent below the average ice extent for 1979 to 2000 and the lowest June ice cover since records began in 1979.

Sea-ice loss and rising seas are "overall a clear indication the planet is warming," he added.

Solar Surprise

There were also some surprising elements to the report, scientists say.

For instance, Weather Underground's Masters was struck by how such extreme temperatures persisted despite weak solar activity. Sunspots weaken and strengthen on a roughly 11-year cycle, and Earth tends to be warmer when this activity is at its peak.

"This year we're at the record low minimum for solar activity. The fact we're having record high temperatures is remarkable," Masters said. "Human-caused global warming and El Niño was able to overwhelm" the sun's influence.

NCAR's Trenberth countered that the impacts of sunspots on Earth's global temperature are "unproven" and "miniscule" at best.